When using a coloured Shulker GUI Resourcepack with OptiGUI, the background of Shulkerboxes opened from the hotbar or the inventory doesn't update.
It is the background of the last placed down Shulkerbox, the player opened.

It would serve as a great visual indicator for what shulkerbox you just opened, when having many in the inventory.

QuickShulker will need some extra code to work with OptiGUI 2, I'll open a PR when I'm done writing it.